The present invention relates to a compound liquid crystal indicator screen having at least two liquid crystal cells arranged in tandem directly behind one another in a liquid beam direction and which cells are either simultaneously or independently operated.
The invention is of particular importance for all liquid crystal indicator screens, which are to be visually observed such as displays, picture screens and similar things, and which screens have at least two liquid crystal cells in a tandem arrangement one behind the other. The invention enables obtaining a freedom from parallax for these screens, especially for screens in which the electrodes of each of the cells are in alignment. In such a screen, the cells may be controlled to operate simultaneously or independent of each other. The invention is also important for an indicator screen which has a multi-color indication.
To increase the contrast which is to be obtained, it is known in the art, for instance in the German Offenlegungschrift 2,059,427 to arrange at least two liquid crystal cells behind one another in such a way that the electrodes are aligned with one another and that the light passes through both cells successively. To obtain this arrangement, it is suggested that the two liquid crystal cells are in a common construction.
Since the time period for switch over of the material of a cell between a light conducting state and a transparent state is approximately proportional to the square of the thickness of the liquid crystal layer of the cell and the thickness of this cell determines the light dispersion ability, a construction of two cells one behind the other enables obtaining the desired light dispersion ability without an increase in the switch-over time. Thus, depending upon the construction and the embodiment of liquid crystal cell, a so-called tandem arrangement provides either a multi-color effect, a movement effect, or in the case of single color operation, an essential increase of the contrast without increasing the switching time.
In a proposed practical embodiment for a tandem arrangement embodiment, a normal glass plate, which is provided with aligned electrode patterns, is used to separate the two liquid crystal layers and a light beam which is a bundle of parallel beams is utilized. A light-sensitive recording carrier, such as a film, is exposed by the parallel beams which are modulated as they pass through the two liquid crystal cells. The recordation carrier or film is moved directly along a surface of one of the outer glass plates of the cell and the bundle of parallel beams is directed through the opposite or other outer glass plate substantially perpendicular thereto.
If the proposed embodiment were used with indicator devices which do not operated with bundles of parallel beams but utilize ambient light conditions, it has an essential drawback. This drawback is based on the interfering parallax which is produced due to the thickness of the common center glass plate which separates the liquid crystal cells which are arranged directly one behind the other.